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Community and Q&A

How to insulate a vaulted ceiling

LJoseph | Posted in Energy Efficiency and Durability on

I am an American living in Provence France. We have hot summers and for the most part mild winters. In January, temperature can drop into the upper twenty during the night. It is a dry climate. I have been renovating a house at different times since last April. During the spring of the year I did the outside roof structure. I installed a new wood decking then a product called flexotuille designed to put under a clay tile roof. After a few other project, I have removed the inside ceiling in the den area in order to vault the ceiling in this area. Some of the structural beams where failing and needed to be replaced. The plan is to leave all structural beams exposed and insulate between the rafters which are 15 cm deep. The heat source in the den is a very efficient wood burning stove and no A/C. After reading Martin Holladay article for insulating vault ceilings, l have a concern. In the winter, with warm inside air passing through the insulation and coming in contact with cold air passing through the roof structure. Will this cause condensation with a potential mold problem? All the insulation contractors said this is not a problem and the roof will dry to the outside. I don’t really believe this b/c there is no air flow. A vented roof system is not possible. I am sure closed cell foam is the best option but does fit into the budget after all the other work. Here is some different option. The first is mine and the others are the insulation contractors. They thought my idea for the panels was crazy.

1st option

Can rigid foam XPS panels go between the rafters? I have found a panel that is just a little wider than the space between the rafters. Can the panels touch the bottom of the sheathing? What is the minimum thickness the panels? I was thinking a layer of rigid foam panel 40 mm then a layer rock wool 100 mm. If there is a 1 to 2 cm space between rockwool and sheetrock is that a problem? If XPS panels fit tight against the rafter, is there anything to be done to the joints?

2nd option

1 layer of 100 mm rockwool with a space of 40 mm to the sheathing

3rd option

1 layer of 140 mm rockwool


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  1. GBA Editor
    Martin Holladay | | #1

    The first thing we need to establish is what type of roofing you have. If you have clay tile roofing, then the roofing won't interfere in anyway with outward drying. Clay tiles are vapor permeable, and moisture passes through them easily.

    Can you describe the material you mentioned -- the FlexoTuille? If this product is vapor-permeable, and your roof assembly doesn't include any vapor-impermeable products like synthetic roofing underlayment or Grace Ice & Water Shield (or a similar peel-and-stick product), then the contractors you have spoken to are correct: This type of roof will dry to the exterior.

    [Note to GBA readers: 15 cm. = 5.9 inches.]

  2. LJoseph | | #2

    Flexotuille is vapor-permeable. So, can I use 140cm rockwool in the rafter area and not install XPS?

  3. GBA Editor
    Martin Holladay | | #3

    The more insulation, the better. I don't think you mean 140 cm, however -- that's 55 inches (about 4 1/2 feet).

    You probably mean 140 mm (5 1/2 inches).

    The R-value of 140 mm of rock wool is about R-20. That's OK, but not great. If you want a higher R-value, you can add a continuous layer of rigid foam on the interior side of the rafters.

    Assuming that you have clay tile roofing and that Flexotuille is vapor-permeable, there is no reason to worry about moisture building up in this assembly.

  4. LJoseph | | #4

    Yes, 140 mm and the roof structure is clay tiles. What is your opinion of using the brand Rockwool? What I have read it seem like a good product. What is a good thickness for the rigid foam?

  5. GBA Editor
    Martin Holladay | | #5

    As far as I know, rock wool is a generic designation -- it's also called mineral wool -- not a brand name, although the situation may be different in Europe. Mineral wool performs very well if it is properly installed.

    A continuous layer of interior rigid foam is optional, not required, unless you need to meet some type of French energy code requirement. (I don't know anything about French building codes.)

    If you decide to install a continuous layer of interior rigid foam, the thickness depends on your R-value goal.

  6. LJoseph | | #6

    I really appreciate all your helpful info. Thank you!!!

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